Twins are rare.
Having twins is a rare thing to happen during pregnancy but it's not as rare as having multiples or triplets or more.
Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families.
But there are some factors that make having non-identical twins more likely: non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese.
It is estimated that 1 in 250 natural pregnancies will naturally result in twins.
While twin pregnancies can happen by chance, there are some factors that may increase your odds of having two babies at the same time.
The rarest type of twins are Monoamniotic-monochorionic Twins as these types of twins share a chorion, placenta, and an amniotic sac.
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords.
In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.
And women who are fraternal twins have a 1 in 60 chance of having twins, and men who are fraternal twins have a 1 in 125 chance of fathering twins.